A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol (such as methanol or ethanol), a hydride or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, or other oxidant such as hydrogen peroxide is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of the oxidant at the cathode.
Fuel cells are usually classified according to their electrolyte: proton exchange membrane (PEM) fuel cells including hydrogen (including reformed hydrocarbon fuel) fuel cells, direct methanol fuel cells (DMFC), direct ethanol fuel cells (DEFC), formic acid fuel cells and hydride fuel cells; alkaline electrolyte fuel cells; phosphoric acid fuel cells (including hydrogen or reformed hydrocarbon fuel); solid oxide fuel cells (reformed or unreformed hydrocarbon fuel); and molten carbonate fuel cells (hydrogen and reformed hydrocarbon fuel).
In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but ionically conducting. Proton-conducting membranes are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the solid polymeric membrane. On either side of the membrane there is an electrocatalyst layer, containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer, must allow products to be removed from the electrocatalyst layer, and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
The electrocatalyst layer is generally composed of a metal, (such as a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium and osmium), gold or silver, or a base metal) either unsupported in the form of a finely dispersed metal powder (a metal black) or supported on an electrically conducting support, such as a high surface area carbon material. Suitable carbons typically include those from the carbon black family, such as oil furnace blacks, extra-conductive blacks, acetylene blacks and graphitised versions thereof. Exemplary carbons include Akzo Nobel Ketjen EC300J, Cabot Vulcan XC72R and Denka Acetylene Black. The electrocatalyst layers suitably comprise other components, such as ion-conducting polymer, which is included to improve the ionic conductivity within the layer. The electrocatalyst layers also comprise a certain volume fraction of porosity, which allows reactant ingress and product egress.
During normal PEM fuel cell operation, hydrogen-containing gas is fed to the anode and air to the cathode; however during shut down and start up conditions depletion of hydrogen and an ingress of air to the anode electrode can occur and results in an increase in potential at both electrodes. This so-called ‘reverse current decay mechanism’ can lead to high potentials in excess of 1.2V at the cathode electrode, resulting in electrochemical oxidation (corrosion) and loss of the carbon support. This process leads to a collapse in the catalyst layer structure, a loss of active catalyst metal surface area and irreversible fuel cell performance decay. An operational system will experience repeated start/stops over the lifetime of thousands of hours and therefore repeated excursions to high voltages causing corrosion and associated performance decay. Under normal operating conditions depletion of hydrogen fuel at the anode electrode whilst under load can also lead to carbon corrosion. Prolonged ‘idling’ of the system results in exposure of the cathode electrode to potentials around ˜0.9V which could cause deterioration of the carbon support and catalyst. Operation at higher temperatures up to 120° C. is particularly desirable for automotive PEM fuel cell systems; however increasing temperature also promotes the carbon corrosion process and is therefore likely to accelerate any of the decay mechanisms described.
It is generally the case with the carbon materials used as catalyst support materials for fuel cell applications, that an increase in the total (BET) surface area results in an increase in the catalyst surface metal area, due to the formation of smaller catalyst particles, as measured by ex-situ gas phase chemisorption metal area or also the in-situ electrochemical surface area under fuel cell testing conditions. The increased catalyst surface area is often associated with an increase in the activity of the catalyst in a fuel cell environment. However, an increase in total (BET) surface area of the carbon support invariably corresponds with an increase in the corrosion of the support under fuel cell operating conditions where high potentials occur.
It is possible to improve the corrosion resistance of carbon supports through various processes, particularly high temperature graphitising treatments, but the resultant catalysts have lower active catalyst metal area and thus lower activity compared to the comparable untreated carbon supported catalysts.
It is therefore an object of the present invention to provide an improved catalyst which demonstrates both comparable activity to conventional catalysts, but which is more resistant to corrosion.